The present disclosure relates to the field of waste recovery. In particular, the production of a flowable fill material that incorporates waste materials. More particularly, the disclosure relates to rapid-setting and self-hardening flowable fill material that utilizes both waste wood chips and coal combustion by-products and provides a disposal medium for waste treated wood.
As world populations continue to rise, the demand for food, fuel and other needed resources rises commensurately. Ever-increasing consumption and corresponding production continues to stress global ecosystems at unprecedented levels. These facts act to fortify the compelling notion of recycling and waste reduction. The disclosure contained herein is comprised of recycled and/or recovered materials.
Scrap Wood represents a significant portion of many waste streams. Studies have shown that wood comprises 20-30% of many waste streams entering municipal solid waste landfills and can exceed 30% at construction and demolition debris landfills. In 1998, the EPA estimated that 136 million tons of building-related debris was generated, with wood representing the largest single component. This fact becomes much more concerning when it is considered that a significant portion of the wood currently in use is treated, thus a significant portion of the wood waste stream is comprised of treated wood.
Wood is commonly treated with preservatives to inhibit fungal and microbial decay. Common preservatives include creosote and pentachlorophenol. But the predominant preservative over the past 10-20 years has been chromated copper arsenate (CCA) that introduces copper chromium and arsenic to the wood. Chromium and arsenic and toxic to mammals and copper is toxic to most aquatic life.
During the CCA treatment process, wood is first dried in a kiln. The dried wood is then placed in a pressure vessel where a vacuum is pulled and effectively applied to the wood's pore space. A CCA solution is then introduced into the vessel and pressurized causing the solution to enter the pore spaces. After the pressure is removed, the wood is the removed from the vessel and allowed to dry over a drip pad.
There are three types of CCA-treated wood: Type A, Type B, and Type C, with Type C being the most common. Type C CCA is composed (by weight) of 34.0% As2O5, 47.5% CrO3, and 18.5% CuO.
The American Wood Preservers Institute (AWPI) estimates that treating wood with CCA increases the usable life of wood by 7 to 12 times. The actual CCA content in treated wood is significant. It is estimated that the amount of arsenic associated with CCA-treated wood currently in service in the state of Florida alone is 26,800 tons. Though the desirable characteristics of treated wood are obvious, it has been observed that chemically treating lumber takes a perfectly useable, recyclable material from a renewable resource and renders it toxic.
CCA became widely used in the late 1970s and now represents approximately 80% of the treated wood market in the U.S. It is estimated that 6.5 billion board feet are treated each year in the U.S. Treated wood is particularly well suited for use in residential and small commercial decks, and estimates are that CCA treated wood has been used in over 80% of residential decks since the early 1970s. As the treated wood that is now in service expires in the coming years, the amount of CCA-treated wood arriving at landfills is expected to peak sometime between the years 2020 and 2030.
It is difficult to visually discern between treated and untreated wood, thus most states currently CCA-treated wood in the same manner as other discarded wood and wood products. Therefore, most treated wood in the waste stream ends up at unlined construction and demolition waste landfills. A far lesser amount is disposed in municipal solid waste landfills or incinerated at waste to energy facilities. Since the toxic metals in CCA easily survive the incineration process, burning treated wood is discouraged. Further, incinerating can convert trivalent chromium into highly toxic hexavalent chromium.
As previously discussed, the metals in CCA easily survive incineration and escape with flue gas. Further numerous studies have shown that CCA, particularly the arsenic component easily leaches from treated wood, thus, incineration, landfill disposal and processing into mulch are not suitable waste outlets for CCA-treated wood. Though little data is currently available, there is growing concern about the impact that CCA-treated wood in landfills may ultimately have on groundwater. Given these problems associated with CCA-treated wood and the existing and ever-growing problems associated with disposal or reuse, there exists a substantial need for an economical, safe, and environmentally responsible means of disposing of expired CCA-treated wood.
Concrete is one of the oldest and most important composite materials known to man and generally consists of a course aggregate (rock and/or gravel), sand, and hydrated Portland cement. The finished properties of concrete generally depend on several factors: ratio of cement, sand and aggregate; ratio of water to cement; nature of the course aggregate; mixing and laying methods; and curing time.
Portland cement, named after its likeness to the indigenous limestone of Portland Bill, England, is a mixture of primarily four minerals: tricalcium silicate (3CaO.SiO2); dicalcium silicate (2CaO.SiO2); tricalcium aluminate (3CaO.Al2O); and tetracalcium aluminoferrite (4CaO.Al2O3.Fe2O3). Typical composition, expressd in terms of oxides, is 65% Ca, 20% SiO2, 5% Al2O3, with the balance comprised of Fe2O3 and other admixtures. Portland cement is generally obtained by mixing materials which supply lime (usually limestone or chalk) with materials which supply silica and alumina (usually clay) and firing these mixtures to ˜1200 K for some period of time. Hydration of the oxides in Portland cement is a reaction that proceed for a very long time, as evidenced by concretes continued measurable hardening for years.
Fly ash is the inorganic non-combustible portion of coal that remains after pulverized coal is burned, and is generated in huge quantities by coal-fired electric generating facilities throughout the U.S. Fly ash is comprised of glassy, spherical shaped particles that are typically recovered from flue gas by means of electrostatic precipitators. There are two common Classes of fly ash: Class C and Class F. Class C is produced from burning lignite and sub-bituminous coal, and Class F is produced from burning anthracite and bituminous coal. The comparative typical mineral content of Class C fly ash, Class F fly ash, and Portland cement is shown below in Table 1.
TABLE IChemicalClass CClass FPortlandCompoundFly AshFly AshCementSiO39.9054.9022.60Al2O316.7025.804.30Fe2O35.806.902.40CaO24.308.7064.40MgO4.601.802.10SO33.300.602.30Na2O & K2O1.300.600.60
An important characteristic of Class C fly ash is self-cementing or self-hardening when mixed with water, characteristics enabled by a relatively high CaO (lime) content. As can be seen in Table 1, the CaO content of typical Class C fly ash is almost three times that of Class F.
Fly ash has found particular use as an additive in concrete and pavement materials. When added to Portland cement, fly ash effectively reduces the amount of lime needed and aids in converting free lime to calcium silicate hydrate, a substance similar to the dicalcium and tricalcium silicates—the strongest paste portion of concrete—formed during curing. Concrete enhanced with Class C fly ash will typically strengthen faster than plain concrete due to the additional lime content. Concrete enhanced with Class F fly ash will typically strengthen slower than plain concrete until about 7 days, then it will typically strengthen at a faster rate.
The environmental benefits of using fly ash are numerous. Materials such as Portland cement, lime and crushed stone require energy to produce. Utilizing one ton of fly ash to replace an equivalent ton of any of these materials conserves the energy equivalent of one barrel of crude oil, as well as reducing CO2 emissions by approximately one ton.
A Flowable fill prepared from fly ash, also commonly known as controlled density fill, has many highly desirable properties. It is a fill material that flows easily, is self-leveling, self-compacting, and non-settling after hardening in place. A fly ash-based flowable fill will easily encapsulate whatever has been placed in a trench or void and will provide protection after hardening. A fly ash-based flowable fill can typically be placed in any weather at any time and will displace standing water. Fly ash-based flowable fill can typically be prepared using conventional concrete mixing equipment and pumped using conventional concrete pumping equipment.
The disclosure contained herein takes advantage of the self-cementing properties of Class C fly ash and enhances these properties by the addition of brine to produce a flowable fill material which not only provides a medium for a preferred disposal means for treated wood, but also provides a highly effective fill material useful for a plurality of backfilling applications.